The importance of backspin braking systems in surface pump drive systems for downhole rotary pumps, particularly progressing cavity pump (PCP) drive systems for oil or water wells, is well known in the art. Surface drive systems for PCPs are generally called wellhead drives, drive heads, surface drives or drives.
Wellhead drives for PCPs, at surface, rotate in a forward direction to rotate sucker rods extending down a well which turn a rotor inside a stator at a bottom of the well to pump fluids from the well. When a drive is shut down, energy is released. The energy that is released includes the spring energy stored in the wind-up of the sucker rods and the fluid energy stored in the height difference between the fluid in the production tubing and the fluid in the annulus between the production tubing and the casing. Wellhead drives for progressing cavity pump systems generally include a backspin braking system to control the backspin speed to a safe limit while absorbing and dissipating the energy. The safe speed is determined by the speed rating of the drive head, the power transmission system, or the prime mover.
Commonly, sheaves and belts are used to transmit power from the prime mover to the drive head. If sheaves turn fast enough, such as during uncontrolled backspin, they will shatter due to tensile stresses in the rim resulting from centrifugal forces. Fragments from shattered sheaves are very dangerous to operating personnel. Due to such personnel hazards, backspin braking systems must be designed to be very reliable.
Applicant believes that, until commercial production by Weatherford in 1998 of the centrifugal braking system patented by Hult et al, U.S. Pat. No. 6,079,489, all major manufacturers produced drive heads that required hydraulic pumping to achieve braking. Two major types of braking systems were common; hydraulic and hydraulic actuated. The hydraulic type uses a form of hydraulic pump and restricts output flow in the backspin direction to apply braking torque. Hydraulic type centrifugal braking drives have been produced by Griffin Legrand, Weatherford Corod and Weatherford BMW. The hydraulic-actuated type use a small hydraulic pump to actuate a disc brake in the backspin direction. These types of drives were patented by Mills CA 2,074,013 and U.S. Pat. No. 5,358,036 and are produced by Kudu Industries Inc., Calgary, Alberta, Canada and Weatherford BMW. Since 1998, a hydrodynamic braking system patented by Belcher, CA 2,171,899 was introduced by Corlac Industries of Lloydminster, Alberta, Canada and is now produced by National Oil Well VARCO of Houston, Tex., USA. All braking systems that are based on pumping a fluid hydrostatically or hydrodynamically are vulnerable to failure due to no oil, cold oil or the wrong oil. There are also a number of other modes of failure that are known in the industry based on many years of experience with thousands of fluid pumping based braking systems.
Applicant has provided prior art centrifugal brake systems. Canadian application 2,311,036 described a centrifugal brake referred to as a leading shoe design. Because the brake shoe leads the brake shoe pivot, the friction force between the brake shoe and the brake drum tends to force the brake shoe into the drum. The braking effect using a leading shoe geometry is about 150% of the braking effect based on centrifugal force only, known as a braking multiplier, the principles of which are well understood by those skilled in the art A ball drop principle is used to engage the brake in a backspin direction and disengage the brake in a forward direction. As the engagement system acts on the brake driving hub, the engagement system has no effect on the braking multiplier.
U.S. Pat. No. 6,079,489 to Applicant describes a brake which is actuated by a cam surface built into the actuator hub which actuates on backspin by pushing in a radially outward direction against an inner flat surface of a brake shoe. The line of action of the contact force between the cam and the flat surface is radial and tangential. The tangential component is due to friction between the cam and the flat surface. The cam and the flat surface are oil lubricated plane surfaces and therefore the friction coefficient would be in the order of 0.2 and the line of action of the contact force is approximately 30 degrees off radial. As the cam angle approaches the line of action, the brake multiplier effect gets larger and larger, until the brake locks up. Locking up is not acceptable on a wellhead drive since it is important to let the rods continue to turn and release the energy from the well. In practice, this device is limited to a brake multiplier effect of 2.0 to 2.5.
Ideally a centrifugal braking system is desired which is capable of preventing backspin, is not prematurely engaged, permits dissipation of stored energy without locking up and which is capable of withstanding significant amounts of torque, such as for use in a wellhead drive of a progressive cavity pump.